The present invention relates to systems for positioning and navigation based upon the reception of signals transmitted from satellites orbiting the earth. More particularly, the present invention relates to a system for recovering the carrier frequency from direct sequence spread spectrum satellite signals without knowledge of the said spread spectrum codes and using the carrier frequencies from several geometrically spaced signal sources provided by the Global Positioning System (hereinafter referred to as GPS) to compute the velocity of a balloon borne sonde.
The GPS will consist of eighteen or more satellites in 12 hour circular orbits at a range of approximately 11,000 nautical miles above the earth. The orbital positions of these satellites has been selected to maximize the probability that at least four satellites with suitable geometry for navigation will be in view at all times and positions on the earth. The signals emitted from these satellites are of the same carrier frequency. Each satellite has a different spread spectrum access code for both a clear acquisition (C/A) and a precision (P) code. The C/A code is a pseudorandom string of ones and zeros applied to a device which controls the carrier phase in 180 degree increments. This technique is known as bi-phase direct sequence spread spectrum at a 1.023 MHz rate. The P code is much longer in length and is applied at a 10.23 MHz rate. Details of the NAVSTAR/GPS are given in NAVIGATION: Journal of the Institution of Navigation, Vol. 25, No. 2, December 1978.
The standard GPS navigation receiver must generate a replica of the transmitted code for control of an internal phase switch and synchronize the code in time with the code received at its antenna in order to recover the carrier frequency. The code time with respect to an internal clock is measured for four satellites and used for determining the position of the GPS receiver on the earth. See, for example, U.S. Pat. Nos. 4,457,006 and 4,114,155.
Present windfinding systems make use of inexpensive disposable LORAN and OMEGA receivers installed in sondes which measure barometric pressure, temperature and humidity. See, for example, U.S. Pat. No. 4,445,120. The U.S. Government plans to discontinue the LORAN and OMEGA transmissions some time after the GPS navigation satellites are placed in orbit. The GPS system will provide world wide navigation with better accuracy and lower cost than LORAN or OMEGA. The development of a GPS windfinding system is then necessary for continued wind verses altitude measurements. Such systems desirably should have a sonde of minimum cost because it is disposable. There is the need for a receiver which avoids the expense and complexity of code generation and synchronization for the purposes of finding differential positions and velocity.